460 TROPISMS—THE SLAVES OF THE LAMP 



lower organisms. By it, the animal is drawn towards or draws 

 away from certain chemical substances. The organ stimulated 

 asymmetrically is orientated so that the stimulating impacts on it 

 are symmetrical (see Chap. XII.). 



V. Galvanotropism. It is easy to see that a simple animalcule, 

 like amoeba, would come readily under any influence which 

 altered either its surface energy or the distribution of its colloidal 

 contents or both. We have seen how colloids are attracted 

 according to Hardy's Rule {q.v.) to anode or cathode according to 

 which side of their isoelectric point the /jH of the medium lies. 



The isoelectric points of all body proteins (Table XIV.) are on 

 the acid side of pH 7, and, therefore, if free to move, they would 

 tend to collect on the side of the cell nearest the anode (p. 92). 

 Either this produces some changes in the tension of the cell (cf. 

 geotropism) or the lipoids or some other factors unknown as yet 

 come into play and produce movement towards the cathode. 

 Various experiments indicate that the former hypothesis is, at 

 least, plausible. 



VI. Orientation in space is determined mainly by three factors, 

 liglit, tactile sense and gravitation. Normal equilibrium or normal 

 geotropic orientation is defined as that position in which the plane 

 of synmietry of the animal passes through the centre of the earth. 

 Any deviation from that position causes unilateral stimulation 

 and corrective movements are instituted. The tight-rope walker 

 perceives that his centre of gravity is tending towards unstable 

 equilibrium and, voluntarily (though generally subconsciously), 

 corrects his balance. In the labyrinths, we have a delicate 

 mechanism for detecting alterations in our orientation in space 

 {q.v.). 



Crozier and his co-workers have applied simultaneously two 

 types of stimuli to which their experimental animals were sensitive. 

 For example, negatively phototropic animals which when left in 

 the dark would follow a path perpendicularly straight up a vertical 

 plate (negatively geotropic) were subjected to a series of rays of 

 light applied at right angles to and in the same plane as their path. 

 The result invariably was a deviation of the path through an angle 

 which was constant for that type of animal for each intensity of 

 light. One can then standardise the geotropic intensity as equal 

 to the effective light intensity when the angle of deviation is 

 45 degrees from the perpendicular. 



In the same way one could apply any two other types of stimula- 

 tion, and so on till all types of stimulation were standardised as to 

 their tropic intensities in terms of one another. One would then 

 be able to predict the angle of deviation of the path of any tropic 



